Oscillator circuits—whose basic principle is based on the oscillator circuit described in Tietze, Schenk: “Halbleiter-Schaltungstechnik” [Semiconductor Circuitry], 11th edition, Springer Verlag, page 914, FIG. 14.19—are known generally.
Such a known arrangement will first of all be explained with reference to FIG. 1 in order to assist understanding of the invention explained below. The oscillator circuit includes a resonator Q1, particularly a crystal resonator, which is connected between a terminal K1 and a connection for a negative supply potential VSS or reference-ground potential. A variable capacitor Cs is connected in series with the resonator Q1 for the purpose of setting or trimming the resonant frequency.
A drive circuit having a current source circuit 10 with positive feedback is provided for the purpose of exciting the resonator, the current source circuit 10 being connected between a positive supply potential VDD and the terminal K1. In addition, the drive circuit comprises a current sink Iq1 which is connected between the terminal K1 and the negative supply potential VSS and is used to set a basic current that is supplied by the current source circuit 10.
The current source circuit 10 includes a first transistor M1 which, in the example, is in the form of an NMOS transistor and whose load path is connected in series with a first resistor R1 between the positive supply potential VDD and the first terminal K1, a capacitor C1 being in parallel with the first resistor R1. This first transistor M1 is operated with positive feedback to a current Iosc flowing into or out of the resonator Q1, as will be explained below. Provided for this purpose is a second transistor M2 which, in the example, is likewise in the form of an NMOS transistor whose control connection is connected to a node that is common to the load path of the first transistor M1 and the first resistor R1. The load path of this second transistor M2 is connected in series with a second resistor R2 and is connected to the positive supply potential VDD via this second resistor R2. A second capacitor C2 is in parallel with this second resistor R2, the drive connection of the first transistor M1 being connected to a node that is common to the load path of the second transistor M2, the second capacitance C2 and the second resistor R2.
The second transistor M2 is part of an amplifier circuit which is in the form of a differential amplifier and, in addition to the second transistor M2, comprises a third transistor M3 whose load path is connected in series with a third resistor R3 in parallel with the series circuit including the second transistor M2 and the second resistor R2. A node that is common to the load paths of the second and third transistors M2, M3 is connected, via a second current source Iq2, to the negative supply potential VSS. For the purpose of setting the operating point of the differential amplifier and the third transistor M3, a fourth resistor is connected between the positive supply potential VDD and the drive connection of said third transistor, and a third current source Iq3 is connected between the drive connection of said third transistor and the negative supply potential VSS.
In the steady state, the resonator Q1 draws a periodic current Iosc whose frequency corresponds to the resonator's resonant frequency and whose DC component is zero.
In the circuit arrangement having the current source 10, the differential amplifier M2, M3 and the setting circuit R4, Iq3, the latter are matched to one another in this case in such a manner that, in the case of an oscillator current Iosc=0, the drive potentials P2, P3 for the second and third transistors M2, M3 are identical when the current I1 flows through the first transistor M. This matching is effected by dimensioning the first and fourth resistors. The second and third resistors are usually the same size.
The way in which the invention described works is explained below.
To this end, the case in which the oscillator current Iosc is equal to zero will be considered first of all. In that case, the current I1 supplied by the current source flows through the first resistor R1, said current giving rise to a voltage dropU1=I1·R1  (1)across this resistor. The drive potential P2 on the drive connection of the second transistor M2 is thenP2=VDD−U1=VDD−I1·R1  (2).
A current I21 which gives rise to a voltage drop U2 across the second resistor R2 flows through the second transistor M2, said voltage drop determining the drive potential P1 for the first transistor M1, for whichP1=VDD−U2  (3).
If a current Iosc now flows, in the direction indicated, from the terminal K1 into the resonator Q1, the current flowing through the first transistor M1 rises by this current drawn by the resonator Q1. As a result, the voltage drop U1 across the first resistor R1 rises, and the drive potential P2 for the second transistor M2 falls. This limits the second transistor M2, as a result of which the current flowing through the resistor R2 falls and the voltage drop U2 across this resistor R2 decreases. This increases the drive potential P1 for the first transistor M1, as a result of which the transistor M1 is turned on in order to increase the current flowing into the resonator Q1.
If, by contrast, a current flows from the resonator Q1 into the terminal K1, the current flowing through the first transistor M1 is reduced by the current Iosc provided by the resonator Q1. As a result, the voltage drop U1 across the first resistor R1 falls, and the drive potential P2 for the second transistor M2 rises. This turns on the second transistor M2, as a result of which the current flowing through the resistor R2 rises, and the voltage drop U2 across this resistor R2 increases. As a result, the drive potential P1 for the first transistor M1 falls, as a result of which this first transistor M1 is limited to the previous operating point in order to increase the current flowing from the resonator Q1 into the terminal K1.
In summary, the current source arrangement 10 is thus operated with positive feedback to the current which is drawn by the resonator and varies periodically at the resonant frequency.
The capacitors C1, C2 which are connected in parallel with the first and second resistors R1, R2 are dimensioned in such a manner that the oscillator is stimulated to oscillate at its fundamental frequency but not at harmonics of the resonant frequency. Changes in the currents flowing through the first and second transistors M1, M2, at a frequency above the resonant frequency, are filtered out by these capacitors C1, C2 and can thus change the respective drive potentials P1, P2 for the transistors M1, M2 to a lesser extent.
The following is true for an input impedance Zinl of the drive circuit at the terminal K1:Zin=1/gm1−Z1·Z2·gm2/2  (4),where gm1 denotes the transconductance, i.e., the ratio of the output current to the applied voltage, of the first transistor M1 at the operating point at which the transistor is operated. gm2 accordingly denotes the transconductance of the second transistor. Z1 denotes the impedance of the parallel circuit including the first resistor R1 and the first capacitor, and Z2 denotes the impedance of the parallel circuit including the second resistor R2 and the second capacitor C2. Referring to the equivalent circuit diagram which is likewise illustrated in FIG. 1, this input impedance is illustrated as a series circuit including a negative resistance and an inductance.
Interference may occur in such an oscillator arrangement when radio-frequency noise signals (EMI signals) are injected into the circuit at the terminal K1 and are superimposed on the oscillator current Iosc.
In order to increase the robustness of such an oscillator arrangement with respect to radio-frequency noise signals, it is possible to increase the basic current I1 of the first current source Iq1. However, such a procedure is not suitable for oscillator arrangements which are used in systems that have been optimized for a low power consumption.